Field of the Invention
This disclosure is related to the field of polymer interlayers for multiple layer glass panels and multiple layer glass panels having at least one polymer interlayer sheet. Specifically, this disclosure is related to the field of polymer interlayers comprising multiple thermoplastic layers which resist the formation of optical defects.
Description of Related Art
Multiple layer panels are generally panels comprised of two sheets of a substrate (such as, but not limited to, glass, polyester, polyacrylate, or polycarbonate) with one or more polymer interlayers sandwiched therebetween. The laminated multiple layer glass panels are commonly utilized in architectural window applications and in the windows of motor vehicles and airplanes, and in photovoltaic solar panels. The first two applications are commonly referred to as laminated safety glass. The main function of the interlayer in the laminated safety glass is to absorb energy resulting from impact or force applied to the glass, to keep the layers of glass bonded even when the force is applied and the glass is broken, and to prevent the glass from breaking up into sharp pieces. Additionally, the interlayer may also give the glass a much higher sound insulation rating, reduce UV and/or IR light transmission, and enhance the aesthetic appeal of the associated window. In regard to the photovoltaic applications, the main function of the interlayer is to encapsulate the photovoltaic solar panels which are used to generate and supply electricity in commercial and residential applications.
In order to achieve the certain property and performance characteristics for the glass panel, it has become common practice to utilize multiple layer or multilayered interlayers. As used herein, the terms “multilayer” and “multiple layers” mean an interlayer having more than one layer, and multilayer and multiple layer may be used interchangeably. Multiple layer interlayers typically contain at least one soft layer and at least one stiff layer. Interlayers with one soft “core” layer sandwiched between two more rigid or stiff “skin” layers have been designed with sound insulation properties for the glass panel. Interlayers having the reverse configuration, that is, with one stiff layer sandwiched between two more soft layers have been found to improve the impact performance of the glass panel and can also be designed for sound insulation. Examples of multiple layer interlayers also include the interlayers with at least one “clear” or non-colored layer and at least one colored layer or at least one conventional layer, e.g., non-acoustic layer, and at least one acoustic layer. Other examples of multiple layer interlayers include interlayers with at least two layers with different colors for aesthetic appeal. The colored layer typically contains pigments or dyes or some combination of pigments and dyes. The layers of the interlayer are generally produced by mixing a polymer resin such as poly(vinyl butyral) with one or more plasticizers and melt processing the mix into a sheet by any applicable process or method known to one of skill in the art, including, but not limited to, extrusion, with the layers being combined by processes such as co-extrusion and lamination. Other additional ingredients may optionally be added for various other purposes. After the interlayer sheet is formed, it is typically collected and rolled for transportation and storage and for later use in the multiple layer glass panel, as discussed below.
The following offers a simplified description of the manner in which multiple layer glass panels are generally produced in combination with the interlayers. First, at least one polymer interlayer sheet (single or multilayer) is placed between two substrates and any excess interlayer is trimmed from the edges, creating an assembly.
It is not uncommon for multiple polymer interlayer sheets or a polymer interlayer sheet with multiple layers (or a combination of both) to be placed within the two substrates creating a multiple layer glass panel with multiple polymer interlayers. Then, air is removed from the assembly by an applicable process or method known to one of skill in the art; e.g., through nip rollers, vacuum bag or another deairing mechanism. Additionally, the interlayer is partially press-bonded to the substrates by any method known to one of ordinary skill in the art. In a last step, in order to form a final unitary structure, this preliminary bonding is rendered more permanent by a high temperature and pressure lamination process, or any other method known to one of ordinary skill in the art such as, but not limited to, autoclaving.
Multilayer interlayers such as a trilayer interlayer having a soft core layer and two stiffer skin layers are commercially available. The stiff skin layers provide handling, processing and mechanical strength of the interlayer; the soft core layer provides acoustic damping property. Glass panels containing these multilayered acoustic interlayers can, under extreme conditions, develop defects commonly known as iceflowers (also known as snowflakes), which initiate in the presence of excessive residual, trapped air in the panels and stress in the glass. Specifically, during the manufacturing process of laminated multiple layer glass panel constructs, air and other gasses often become trapped in the interstitial spaces between the substrates and the interlayer or between the individual layers of the multilayered interlayer when these layers are stacked together to form the multilayered interlayer. As noted above, this trapped air is generally removed in the glazing or panel manufacturing process by vacuum or nip roll de-airing the construct. However, these technologies are not always effective in removing all of the air trapped in the interstitial spaces between the substrates. These pockets of air are particularly evident with mismatched glass (e.g., tempered glass, heat strengthened glass, and thick, annealed glass) and in windshields, where the curvature of the glass generally results in gaps of air. These gaps of air in windshields are commonly referred to as “bending gaps.” Additionally, when a bending gap is present during autoclaving, heat and pressure compress the glass to conform to the interlayer and narrow the gap, resulting in high stresses in the glass in the original gap area.
As noted above, the de-airing technologies are not always effective in removing all of the air from the glass panel assembly. As a result, there is residual air present between the glass and interlayer. During autoclaving, the residual air dissolves into the interlayer, mostly in the skin layer, under heat and pressure. The residual air located in the skin layer can move into the core layer or skin-core interphase, and it eventually partitions between skin layer and core layer to reach an equilibrium state. When a large amount of residual air (e.g., excessive residual air) is present in the interlayer, air bubbles can nucleate, especially at high temperatures, as the interlayer becomes soft and is less resistant to the nucleation.
With multilayer acoustic interlayers having a soft core layer sandwiched by two stiffer skin layers, e.g., the soft layer is constrained between two stiffer layers, air bubbles commonly first form within the soft core layer as nucleation favors the less viscous medium. In warm to hot climates, such as during the summer season, the temperature of glass can elevate to 50° C. to 100° C. in the laminated glass installed on buildings and vehicles. At these elevated temperatures, forces due to stresses in glass panels or windshields exert pressure on the glass perpendicularly to their plane and in the opposite direction, pulling the glass panels away from each other in an effort to restore them to their original states. The stress reduces the resistance of the air to nucleate and expand and allows the bubble to grow within the core layer.
Bubbles can also nucleate and expand in the skin layers. Because of the relative higher stiffness of the skin layer at the temperature the bubble nucleates and expands and the larger volume of the layers, the bubble expands spherically. When the bubble further expands, it can expand into the core layer, where the expansion of the bubble becomes less resistant. Regardless of where the bubbles initially form, at elevated temperatures (e.g., 50° C. to 100° C.), the stresses from the bending gap or glass mismatch cause the bubbles to expand in the path of least resistance in random radial directions within the core layer. As the defects continue radial expansion, branches and dendritic-like features form, and give the undesirable optical appearance of iceflowers. Additionally, the formation of iceflowers within the core layer typically leads to a separation between the layers, reducing the structural integrity of the panel.
Accordingly, there is a need in the art for the development of a multilayered interlayer that resists the formation of these optical defects without a reduction in other optical, mechanical, and acoustic characteristics of a conventional multilayered interlayer. More specifically, there is a need in the art for the development of multilayered interlayers having at least one soft core layer that resists air nucleation and expansion to form iceflowers while also having superior acoustic properties.